It is often necessary or desirable to measure the cardiac output of a patient. A common technique for accomplishing this is to inject a known volume of an injectate, such as a saline solution, into the right atrium. The injectate has a known temperature and it mixes with the blood to produce a temperature drop in the blood. The temperature of the blood is monitored at a suitable location downstream of the right atrium, and the data obtained can be used to determine cardiac output. This technique is commonly referred to as thermodilution, and the injection of the injectate and the downstream temperature measurement are carried out by a thermodilution catheter.
It is known to obtain the upstream temperature measurement of the injectate using a thermistor permanently mounted in the conduit which supplies the injectate. Although this system functions satisfactorily, it is necessary that the thermistor be initially sterile and be discarded after each usage. This increases the cost of using the thermodilution technique because the thermistor is a relatively expensive component. In addition, the thermistor bead material is subject to being attacked by the injectate.
The injectate may be at room temperature or at reduced temperatures. In the reduced temperature system, the injectate is cooled at one location and manually transported to the injection location near the proximal end of the thermodilution catheter. With this system, there is a risk of loss of sterility, the injectate may not be as cold as desired and there is an absence of a constant, ready supply of the injectate.
This invention provides for shielding the thermistor from the injectate and permits the thermistor to be reused even though other portions of the system may be disposable. This reduces the cost of the system, and the isolation of the thermistor from the injectate prevents the injectate from attacking the material of the thermistor bead. Furthermore, manual transport of the injectate is eliminated by providing a closed system in which a conduit carrier the cold injectate from a cooling container to the location where injection is to be carried out.
Although the concepts of this invention are particularly adapted for use in an injectate delivery system, in a broader sense, the concepts of the invention are also applicable to a system for sensing a characteristic of fluid flowing to or from the body of a human or animal. For example, the characteristic being sensed may be temperature, pressure or any characteristic that can be determined by an optical scan, such as the partial pressure of blood gases. This sensing can be carried out on any fluid, i.e., liquid or gas or mixture thereof, which is being injected into the body or being received from the body.
More particularly, the invention can be embodied in a system which includes a conduit having a first end adapted to be outside the body, a second end adapted to be received within the body and a flow passage through which fluid can flow between the first and second ends. The system also includes a probe, including means for sensing the desired characteristic of the fluid. Means is provided on the conduit for receiving at least the sensing means of the probe from the exterior of the conduit in the flow passage and isolating the probe from the fluid flowing through the flow passage. The probe can be mounted on the conduit with the sensing means in the flow passage and isolated from the fluid flowing in the flow passage so that the probe can sense the desired characteristic of the fluid in the conduit.
With this arrangement, the sensing means is isolated from the fluid flowing through the flow passage. For example, the sensing means may include a thermistor or fiber optics which can scan the fluid in the flow passage. Because the sensing means is isolated from the fluid in the flow passage, the probe need not be sterile. By removably mounting the probe on the conduit, at least the portion of the conduit having the receiving means can be disposed of without disposing of the probe.
The receiving means can be of various different constructions. For example, the receiving means may include a receiver projecting into the flow passage and having a receiver passage opening to the exterior of the flow passage for receiving the sensing means of the probe. Alternatively, the conduit may have a wall with a port leading to the fluid passage, and in this event, the receiving means may include a resilient membrane closing the port. The resilient membrane is deformable by the probe to permit at least the sensing means of the probe to be received in the flow passage.
The means for mounting the probe on the conduit preferably includes elongated telescoping members on the conduit and the probe, respectively. The telescoping members rigidly mount the probe and guide the sensing means into the receiving means.
When the system is used as an injectate delivery system, the conduit preferably has an inlet for receiving the injectate and an outlet through which the injectate can be delivered to the body. In this event, at least a downstream portion of the conduit includes a catheter for delivering the injectate to the interior of the body.
When the system is used to measure temperature, the projection of the receiving means into the flow passage creates turbulence adjacent the receiving means which assists heat transfer. To further increase heat transfer, the fluid passage can be restricted at the receiving means to increase the velocity of the injectate.
When using the system as a cold injectate system, the system preferably includes a cooling container, and the conduit leads from the receiving means to the cooling container with the cooling container being between the inlet and the receiving means. This provides a closed sterile system for the transfer of the cold injectate. In a preferred construction, the conduit includes several coils within the cooling container, and the storage volume within the cooling container is sufficient to store enough cold injectate for multiple injections. To reduce heat transfer to the injectate flowing from the cooling container, a length of the conduit downstream of the cooling container has a relatively low coefficient of heat transfer.
The injectate is forced through the conduit by a suitable pump, such as a syringe. By way of example, the conduit may include a first section coupled to the pump and extending past the receiving means and toward the outlet and an inlet section joined to the first section and leading to the inlet. A check valve is used for substantially preventing flow from the pump through the inlet section in a direction toward the inlet.
The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawing.